Contrary to what might have been supposed, the diameter of the passage increased as the explorers proceeded, so that they very soon were able to stand upright. The granite, worn by the water for an infinite time, was very slippery, and falls were to be dreaded. But the settlers were all attached to each other by a cord, as is frequently done in ascending mountains. Happily some projections of the granite, forming regular steps, made the descent less perilous. Drops, still hanging from the rocks, shone here and there under the light of the torches, and the explorers guessed that the sides were clothed with innumerable stalactites. The engineer examined this black granite. There was not a stratum, not a break in it. The mass was compact, and of an extremely close grain. The passage dated, then, from the very origin of the island. It was not the water which little by little had hollowed it. Pluto and not Neptune had bored it with his own hand, and on the wall traces of an eruptive work could be distinguished, which all the washing of the water had not been able totally to efface.

The settlers descended very slowly. They could not but feel a certain awe, in this venturing into these unknown depths, for the first time visited by human beings. They did not speak, but they thought; and the thought came to more than one, that some polypus or other gigantic cephalopod might inhabit the interior cavities, which were in communication with the sea. However, Top kept at the head of the little band, and they could rely on the sagacity of the dog, who would not fail to give the alarm if there was any need for it.

After having descended about a hundred feet, following a winding road, Harding who was walking on before, stopped, and his companions came up with him. The place where they had halted was wider, so as to form a cavern of moderate dimensions. Drops of water fell from the vault, but that did not prove that they oozed through the rock. They were simply the last traces left by the torrent which had so long thundered through this cavity, and the air there was pure though slightly damp, but producing no mephitic exhalation.

"Well, my dear Cyrus," said Gideon Spilett, "here is a very secure retreat, well hid in the depths of the rock, but it is, however, uninhabitable."

"Why uninhabitable?" asked the sailor.

"Because it is too small and too dark."

"Couldn't we enlarge it, hollow it out, make openings to let in light and air?" replied Pencroft, who now thought nothing impossible.

"Let us go on with our exploration," said Cyrus Harding. "Perhaps lower down, nature will have spared us this labor."

"We have only gone a third of the way," observed Herbert.

"Nearly a third," replied Harding, "for we have descended a hundred feet from the opening, and it is not impossible that a hundred feet farther down--"

"Where is Top?" asked Neb, interrupting his master.

They searched the cavern, but the dog was not there.

"Most likely he has gone on," said Pencroft.

"Let us join him," replied Harding.

The descent was continued. The engineer carefully observed all the deviations of the passage, and notwithstanding so many detours, he could easily have given an account of its general direction, which went towards the sea.

The settlers had gone some fifty feet farther, when their attention was attracted by distant sounds which came up from the depths. They stopped and listened. These sounds, carried through the passage as through an acoustic tube, came clearly to the ear.

"That is Top barking!" cried Herbert.

"Yes," replied Pencroft, "and our brave dog is barking furiously!"

"We have our iron-tipped spears," said Cyrus Harding. "Keep on your guard, and forward!"

"It is becoming more and more interesting," murmured Gideon Spilett in the sailor's ear, who nodded. Harding and his companions rushed to the help of their dog. Top's barking became more and more perceptible, and it seemed strangely fierce. Was he engaged in a struggle with some animal whose retreat he had disturbed? Without thinking of the danger to which they might be exposed, the explorers were now impelled by an irresistible curiosity, and in a few minutes, sixteen feet lower they rejoined Top.